The present invention relates optical communications, and, more particularly, to spectrum aware rate selection for optical channels in flexible wavelength division multiplexing WDM.
In conventional wavelength division multiplexing (WDM) optical networks, the spectrum allocation to the WDM channels (determined from the channel spacing) is fixed, and remains the same throughout the network operations. These channels are centered on standard ITU-T channel grid such as specified according to ITU-T standard G.694.1 [ITU-T]. We refer to such networks as the fixed grid optical WDM networks. In fixed grid networks, the fixed amount of spectrum is assigned to all connections irrespective of their data rates, which may lead to an inefficient utilization of spectral resources (FIG. 1(a)). Such a network is rigid and cannot provide optimum spectral efficiency.
Envisioning the requirement for higher spectral efficiency to support the future traffic volume, there has been several efforts for relaxing the constraint of fixed spectral allocation in optical WDM networks, which we refer as Flexible optical WDM networks (FWDM). The FWDM networks consist of optical channels supporting heterogeneous line rates using variable amounts of spectrum as shown in FIG. 1(b) as opposed to fixed grid networks.
A flexible spectrum assignment in FWDM networks improves spectral efficiency by avoiding over-provisioning of spectral resources for the sub-wavelength granularity traffic and guard bands between multiple channels used to support the super-wavelength granularity traffic compared to fixed grid networks. For example, instead of allocating 50 GHz of spectrum to a channel with 10 Gb/s line rate as in fixed grid networks, 25 GHz of optimum spectrum is allocated to the channel in FWDM networks. On the other hand, instead of establishing four 100 Gb/s channels using 200 GHz of spectrum (including guard bands) for 400 Gb/s data rate, a single channel of 400 Gb/s line rate can be established within continuous 75 GHz of spectrum by eliminating guard bands. On the other hand, due to the flexibility in spectrum allocation, a control plane in FWDM networks must observe additional (1) spectral continuity constraint which is defined as an allocation of the same amount of spectrum on each link along the route, and (2) spectral conflict constraint which is defined as non-overlapping spectrum allocation to channels routed over the same fiber, along with the conventional (3) wavelength continuity constraint which is defined as an allocation of spectrum at the same center wavelength over all links along the route while provisioning channels. In a dynamic traffic scenario, statistical arrivals and departures of channels with heterogeneous spectral requirements leads to spectral fragmentation that partitions the continuous spectral band into smaller spectral-islands as shown in FIG. 2. channel may be blocked over a fiber in spite of an availability of sufficient amount of spectrum for the channel if the available spectrum is fragmented and not continuous.
Additionally, this channel blocking further increases in the network due to the observance of the wavelength continuity, spectral continuity, and spectral conflict constraints if the channel is routed over multiple fragmented fibers. To alleviate the blocking in such a fragmented network, one of the solutions is to split the requested data rate into multiple low-rate channels based on the availability of spectrum over fibers and the offered line rates in the network. Thus, the problem is that for a given requested data rate and given spectrum availability profile of each link in the network, how to select a set of channels such that the total line rate offered by the set of channels meets or exceeds the requested rate while minimizing the total spectrum required by the set of channels. We refer this problem as the spectrum-aware channel selection problem.
In FWDM networks, the spectrum profile of a fiber can be continuous or discrete in the frequency domain. Since a continuous spectrum profile may cause significant management and control plane overheads, network operators prefer to maintain a discrete spectrum profile with sufficient granularity such that network performance is not sacrificed. In a discretized spectrum profile, the smallest unit of spectrum is referred to as a wavelength slot. The spectrum of a channel is defined in terms of the number of consecutive wavelength slots. A wavelength slot can be either in an available state or in an occupied state. The state of this discretized spectrum of a fiber connecting nodes i and j is referred to as the spectrum availability profile and denoted as Cijt, where Cijt=1 denotes that a wavelength slot t is available and Cijt=0 denotes that a wavelength slot t is occupied over the fiber (i, j).
The spectrum-aware channel selection problem is formally defined as follows. Consider a physical network topology G(V, E), where V is a set of ROADM nodes and E is a set of fiber links connecting a pair of nodes. The network offers a set of line rates L, and each line rate lεL requires Xl GHz of spectrum. The spectrum availability profile Cijt of each link (i, j)εE is given. We need to find channels for a traffic demand R(s, d, r), where s is a source node, d is a destination node, and r is the requested data rate in Gb/s, such that total required spectrum for these channels is minimum while supporting the requested data rate r.
In fixed grid networks, since the same amount of spectrum is assigned to all channels and the channel center frequency is fixed, the minimum granularity at which spectrum is fragmented is the standardized channel spacing. This channel spacing is used to support a channel with any granularity in the network. Thus, the total required spectrum by a set of channels can be determined as follows.Total Spectrum=Number of channels*Spectrum of a channelSince the spectrum of all channels is fixed and the same, to minimize the total spectrum, we need to minimize the total number of channels required for the given data rate. Thus, the object of the spectrum-aware channel selection problem is equivalent to minimize the total number of channels to support the data rate of a traffic demand. The spectrum-aware channel selection problem in fixed grid networks can be trivially solved by selecting channels with the maximum line rate, which minimizes the total number of channels for the given data rate.
However, since the spectrum assignment to channels in FWDM networks is flexible, spectrum is fragmented at any granularity; and thus, the channel selection problem becomes more general than in fixed grid networks. The solution of the spectrum-aware channel selection problem in fixed grid networks may not be the solution of the problem in FWDM networks.
Applicants have proposed previously, a rate selection procedure to determine line rates of channels for the requested data rate in FWDM networks. This procedure finds an optimum set of line rates for channels such that the required spectrum is minimized. However, this rate selection procedure does not take into account spectral-islands of the network while selecting line rates for channels. Thus, if the fragmented spectrum profiles of fiber links along the route do not have spectral-islands of sizes at least larger than the spectrum required by a channel, then the channel ends up being blocked. Thus, a selection of line rates without using the information of spectrum availability may lead to higher network blocking.
Accordingly, to avoid the aforementioned blocking, there is a need for a spectrum-aware rate selection procedure that finds a set of line rates for channels based on spectral-islands in the network with the goal of minimizing the required spectrum.